What's the story behind the drawings on the wall in Portal 2?

The drawings on the walls were done by the Rat Man (real name Doug Rattmann), a schizophrenic former employee of Aperture Science. He was convinced GLaDOS would continue to try to kill Aperture employees if switched on, and so took precautions when GLaDOS was 'fixed' and reactivated, thereby surviving GlaDOS flooding the Enrichment Centre with a deadly neurotoxin. He manipulated the order GlaDOS chose test subjects to put Chell first, as Aperture/GlaDOS had identified her as being abnormally tenacious and thus unsuitable for testing (hence the bell curve); his hope was that she'd be more capable of frustrating GlaDOS than he was.

The images you see around the portal gun are a retelling of the events in Portal; Aperture turns GlaDOS on; GlaDOS floods the Enrichment Centre with a deadly neurotoxin; Chell resists GLaDOS's cakey temptation and destroys her, and is put into suspended animation.

Most of his handiwork is from after his pills ran out. Not all of it is logical.

Edit: also, see acmshar's answer for the significance of the equations the Rat Man's scrawled on the walls.

Merus' answer is great for covering the reason for the writing (and especially for the third image). I want to clarify what some of the equations are in the second image.

From left, the first equation (starting with t, and including the equation below it starting with gamma) is referencing Time Dilation (Special relativity).

The equation above that is Schrodinger's Equation for a Single Particle in a Potential Well.

The bottom right equations are Maxwell's Laws

The right hand equation is a wave-particle's momentum

The bottom left equation is the time independent Schrodinger equation.

They're drawings/"instructions" by the "Rat-man", left for Chell when she wakes up.

The full comic is available at http://www.thinkwithportals.com/comic/